Electrostatic recording paper and method of making

ABSTRACT

ELECTROSTATIC RECORDING PAPER IS MADE BY APPLYING THREE SUCCESSIVE AQUEOUS COATES TO THE MACHINE GLAZED SIDE OF A PAPER WEB. THE FIRST COAT CONTAINS TITANIUM DIOXIDE AND AN ELECTROCONDUCTIVE WATER DISPERSIBLE POLYMER OF A VINYLBENZYL QUATERNARY AMMONIUM COMPOUND. THE SECOND COAT IS OXIDIZED STARCH AND CALCIUM CARBONATE. THE THIRD COAT CONTAINS CALCIUM CARBONATE AND A CARBOXYLATED POLYVINYL ACETATE IN AMMONIACAL SOLUTION. THE WEB IS DRIED BETWEEN SUCCESSIVE COATINGS AND MAY BE FINALLY STEAM TREATED.

United States Patent O 3,759,744 ELECTROSTATIC RECORDING PAPER AND METHOD OF MAKING Leonard J. Schliesman, Wisconsin Rapids, Wis., assignmto Consolidated Papers, Inc., Wisconsin Rapids, Wis. No Drawing. Filed Aug. 26, 1971, Ser. No. 175,323

Int. Cl. G03g 5/00 U.S. Cl. 117218 21 Claims ABSTRACT OF THE DISCLOSURE Electrostatic recording paper is made by applying three successive aqueous coats to the machine glazed side of a paper web. The first coat contains titanium dioxide and an electroconductive water dispersible polymer of a vinylbenzyl quaternary ammonium compound. The second coat is oxidized starch and calcium carbonate. The third coat contains calcium carbonate and a carboxylated polyvinyl acetate in ammoniacal solution. The web is dried between successive coatings and may be finally steam treated.

BACKGROUND OF THE INVENTION This invention relates to a composite paper product suitable for electrostatic printing, and a method of making such product. More particularly, this invention relates to a coated electrographic paper wherein all coatings comprise an aqueous medium and are applied by conventional coating equipment.

Electrographic printing broadly comprises the formation of a latent electrostatic image on a suitable sheet of material, followed by development of the image by application of a toner. In one form of electrographic printing, a paper web having a dielectric or insulating surface, while being carried upon a grounded support, is passed into close proximity with styli that may be electrically energized as desired. Energizing of selective styli results in a corresponding group of charged spots on the surface of the paper, whereby the desired image is formed. The charged spots are then treated or developed with an oppositely charged, colored resinous powder dispersed in a volatile solvent, which selectively clings to the charged areas. Heat or pressure is then applied to the paper, thereby fusing the resinous powder to the paper and forming a permanent print.

Previously, electrographic recording paper has been made by applying a dielectric coating on a relatively conductive sheet. Various compounds, such as salts and other compounds capable of retaining or attracting conductive moisture in the sheet may be incorporated into the paper to enhance conductive properties. In some cases, the conductive layer is applied on one side of the sheet and the dielectric is applied to the other side. In other cases, the dielectric is applied over the conductive layer.

Some types of electrographic paper have previously been made by applying a dielectric film of plastic material, such as polyethylene or polystyrene to the paper surface, either by extrusion or by deposition from a volatile organic solvent, such as toluene. These methods have been found to be impractical because of high expense, and because of the difliculties, including fire hazards and toxicity, of using a volatile solvent in a coating operation.

More recently, several proposals have been made to apply both a base conductive coating and a top dielectric coating on paper from an aqueous medium. For example, the patents to Silvernail, No. 3,011,918 and to Gess, Nos. 3,264,137 and 3,348,970, employ a water soluble or dispersible vinylbenzyl quaternary ammonium compound as a conductive agent. Some preferred forms, for example, are polyvinyl benzyl trimethylammonium chlo- 3,759,744 Patented Sept. 18, 1973 'ice ride, (N,N-dimethyl-N-benzylamino ethyl acrylate chloride), polyvinyl benzyl dimethyl ethyl ammonium chloride, and alkyl polyethoxyethanol benzyl ammonium chloride.

In addition, proposals have been made to apply a dielectric coating in an aqueous medium. For example, in the patent to Doggett et al. No. 3,110,621, the proposal is made to utilize a carboxylated vinyl acetate in an aqueous ammoniacal solution as a dielectric coating, with the dielectric initially consisting of a resinous copolymer of vinyl acetate with a small amount of crotonic acid. In order to maintain the proper level of resistivity in the dielectric coating, however, the use of mineral pigments is not advised. Additionally, Doggett et al. disclose that the paper may be coated with a non-wetting clay and wax composition prior to application of the dielectric coat. Coatings containing Wax, however, are difiicult to apply with conventional commercial equipment. Moreover, intermediate coatings containing wax are not necessarily compatible with the final dielectric coat and delamination may occur.

Thus, most, if not all of the foregoing proposals suffer from the common disadvantage of being unsuitable for adaption to reliable commercial production. In applying successive aqueous coats of conductive and dielectric materials to a web, difficulties have been experienced in preventing interaction between the coats during the coating operation, the result is that the coated web may have porous spots to which the resinous toner may cling. Also, the final product may exhibit insufficient toner solvent holdout, such that the toner will be absorbed by capillary action and cause a dark spot on the reverse side of the web. Moreover, the surface produced by application of vinyl acetate alone is objectionally gloss and may have poor handling qualities. Additionally, the dielectric finish usually does not have sufficient tooth or roughness to enable clear writing with a pen or pencil on its dielectric surface.

SUMMARY OF THE INVENTION An object of this invention is to provide multiple coat electrostatic printing paper and method for its production wherein the dielectric coat remains distinct from, and does not interact with, the base coat.

Another object of this invention is the provision of an electrographic paper of low gloss and having smooth surface which is receptive to writing instruments.

A further object of this invention is to provide an improved method for producing electrographic paper wherein all coatings are applied in an aqueous medium and the resulting product has excellent solvent holdout characteristics and good handling properties.

Other objects and advantages of the invention will become apparent from the following description and appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT In connection with electrographic papers comprising a conductive base coat and a dielectric coat applied thereon, I have discovered that the problem of interaction between the base and the dielectric coats may be eliminated by appying an intermediate barrier coating that is compatible with and receptive to the base and dielectric coats and forms a good bond therebetween. In addition, I have discovered that incorporation of calcium carbonate of relatively large particle size into the intermediate and the dielectric coats will result in a paper of low gloss and sufficient tooth for writing qualities, without sacrifice to the electrostatic printing qualities of the paper.

The process of the present invention is preferably carried out on a conventional coating apparatus having means for applying an aqueous mixture of solids onto a web moving at high speeds, as well as means for drying, steaming and winding the coated paper. Although the use of a paper web having a machine finish is suitable for the purposes of the invention, a machine glazed paper, such as that produced on a Yankee machine, is preferred.

The first step of the process comprises applying a conductive base coat to the paper by applying an aqueous mixture of finely divided titanium dioxide and a water soluble conductive compound, such as a vinylbenzyl quaternary ammonium compound, preferably polymerized vinylbenzyl trimethyl ammonium chloride (Dow -ECR-34). The relative proportions of the vinylbenzyl compound and the titanium dioxide are not critical, and a 24 percent solids mixture in water containing from about 30% to about 70% by weight of the vinylbenzyl compound with the remainder as titanium dioxide has been found to give acceptable results. The mixture is applied in a conventional manner by means of a blade coater in weights ranging from about one to about two pounds per ream (3300 sq. ft.) to the glazed or polished surface of the unsized web in order to produce a smooth and even coating. The web is then dried to a moisture content of 3 to 5 percent by passing it through a suitable dryer.

The titanium dioxide in the base coat is beneficial in imparting brightness to the paper and has been found to be fully compatible with the vinylbenzyl trimethyl ammonium chloride, in that conductivity of the deposited mixture is approximately the same as that of the vinylbenzyl compound alone. The vinylbenzyl compound serves primarily as a conductive medium, although it is believed that this compound also acts as a humectant and may form a partial barrier to solvents.

A second coating is then applied over the dried base coat, the purpose of which is to form a relatively impenetrable but fully compatible layer between the conductive base and the dielectric top coating. The layer is preferably applied by a blade coater and consists essentially of an aqueous 40 to 45 percent solids mixture of oxidized or enzyme converted starch, such as a brand known as Clinton-X, and calcium carbonate, preferably Snowflake, calcium carbonate (Thompson, Weinman and Company) or its equivalent having particle sizes within the range of about one to about 24 microns.

The relative proportions of calcium carbonate and starch are not critical and are largely determined by the nature of the coating equipment employed. At relatively high calcium carbonate levels (above 80% by weight), problems such as inadequate solvent holdout and excessive blade wear on blade coaters may occur. At relatively low calcium carbonate levels (below 20% by weight) problems may be encountered in applying the coating to the web by conventional methods. Thus, the preferred relative proportion of calcium carbonate in the coating mixture is within the approximate range of from about 30% to about 70%, with the remainder as starch. The coating weight is preferably maintained within the range of about from two to four pounds per ream, with three pounds being optimum. Preferably, the calcium carbonate has been subjected to little or no surface treatment during processing, which would render this pigment conductive.

The use of calcium carbonate, as compared to clay or other commercial coating agents such as titanium dioxide, has been found to be uniquely suitable for purposes of the present invention. The calcium carbonate not only serves as a bulk carrier for the solubilized starch and provides continuity to the coating, but also is entirely compatible with the other coats and aids in reducing overall gloss, without affecting Charge Acceptance of the product. The starch provides the necessary barrier between the conductive and dielectric coatings and aids in preventing penetration of toner solvents into the web.

After drying, the coated surface of the web is then subjected to a dielectric coating composition consisting essentially f an aq cus mixture of calcium. arbonate and a water soluble dielectric. A suitable dielectric is carboxylated polyvinyl acetate such as that sold as Monsanto CSVlO, dissolved in an aqueous ammoniacal solution Preferably, the aqueous mixture contains 27 to 30 percent solids in proportions of about from 50% to about by weight of polyvinyl acetate with the remainder as Snowflake calcium carbonate. In order to impart the desired tooth or writing qualities to the top coating, proper selection of the particle size of the calcium carbonate is very important and should range in the order of about one to about 24 microns, with a mean particle size of about 5 microns. Sufficient ammonium hydroxide is added to dissolve the polyvinyl acetate or until a pH of about 8.0 to 9.5 has been attained. The coat weight is preferably maintained in the order of from two to three pounds per ream.

Just prior to or after application of the dielectric coating, a base conductive coat may also be applied to the back or unglazed side of the web either by means of a blade or air knife coater. The coat applied is preferably identical in nature and proportions to the base coat previously described and consists essentially of an aqueous mixture of titanium dioxide and polymerized vinylbenzyl trimethyl ammonium chloride. Coating on the back side in this manner has been found to enhance electrostatic printing qualities as well as to minimize possible curl tendencies in the web.

After drying, a second application of the dielectric coating composition may be made on the side previously coated with dielectric for the purpose of assuring uniformity. During the second application, about one to one and one-half pounds of solids per ream are applied, such that a total of about four to four and one-half pounds of dielectric coating is applied to the web. The web is then passed through a final drier and may be wound into a roll.

Between the steps of final drying and rewinding, the moisture level in the web may be increased to about 6 or 7 percent in order to decrease curl tendencies of and cockle in the web, as well as to improve conductivity of the base coat and hence imaging of the product. Moisture is preferably imparted to the web at this stage by passing only the back side of the web through a steam shower.

In further illustration of the invention, the following example is given:

EXAMPLE The machine glazed side of a paper web prepared on a Yankee machine was blade coated with about 1 /2 pounds of solids per ream with an aqueous mixture containing 28 percent solids consisting of equal parts of titanium dioxide and polymerized vinylbenzyl trimethyl ammonium chloride (Dow ECR-34). The web was then dried to a moisture content of about 3 to 5 percent, and a second coat was applied with a blade coater. The second coat comprised a 40% solids aqueous mixture of two parts of an oxidized starch and one part Snowflake calcium carbonate and was applied in the amount of about 3 pounds of solids per ream. After drying, the back side of the web was coated by an air knife with the same base coat as that applied to the glazed surface of the web. The web was then dried, and a dielectric coat was applied by a blade coater over the second coat on the glazed side. The dielectric coat was a 30% solids, aqueous, ammoniacal mixture containing two and one-half parts of a carboxylated polyvinyl acetate (Monsanto C5V10) to one part Snowflake calcium carbonate and was applied at a rate of about 3 pounds per ream. The web was then dried, and a second coating of about one pound of the dielectric was applied with a blade coater. After final drying, the resulting paper was tested on a Statos 1 Paper Test Recorder (Varian Associates) and found to have a DC print rating of 5 in a scale of 0 to an optimum of 5 and one-half, as well as an AC print rating of in a scale of 0 to an optimum of 5.

Although a preferred embodiment of the inveniton has been described, it will be obvious that modifications thereto may be made by those skilled in the art without departing from the principle and scope of the invention as expressed in the appended claims.

I claim:

1. A method of making an electrostatic recording paper comprising the steps of applying a first coat to one side of a web of paper, said first coat consisting essentially of an aqueous mixture of titanium dioxide and a vinylbenzyl quaternary ammonium compound, drying the web, then applying a second aqueous coat over said first coat, said second coat consisting essentially of starch and calcium carbonate, then drying said web, and then applying over said second coat a third coat consisting essentially of an aqueous mixture of calcium carbonate and carboxylated polyvinyl acetate.

2. The method of claim 1 wherein the first coat is applied to the machine glazed side of said web.

3. The method of claim 2 wherein the web is dried after the complete coating thereof and is then subjected to steam on the unglazed side thereof.

4. The method of claim 1 wherein a fourth coat consisting essentially of an aqueous mixture of calcium carbonate and carboxylated polyvinyl acetate is applied over said third coat.

5. The method of claim 1 wherein an aqueous mixture consisting essentially of titanium dioxide and polyvinyl benzyl trimethyl ammonium chloride is applied to the back side of said web.

6. The method of claim 1 wherein the first coat consists essentially of a 28 percent solids mixture of titanium dioxide and polyvinyl benzyl trimethyl ammonium chloride, said second coat contains a 40 to 45 percent solids mixture of oxidized or enzyme converted starch and calcium carbonate, and said third coat contains 27 to 30 percent solids mixture of carboxylated polyvinyl, acetate dissolved in ammoniacal solution and calcium carbonate, the calcium carbonate in said second and third coat having a particle size of from about one to 24 microns.

7. The method of claim 1 wherein the solids of said first coat comprise from about 30% to about 70% of polyvinyl benzyl trimethyl ammonium chloride, with the remainder as titanium dioxide.

8. The method of claim 7 wherein from about one to two pounds per 3300 square feet of said first coat are applied to said web.

9. The method of claim 1 wherein the particle size of the calcium carbonate in said second coat is within the range of about from one to 24 microns.

10. The method of claim 9 wherein the solids of said second coat comprise from about 30% to about 70% calcium carbonate, with the remainder as starch.

11. The method of claim 10 wherein the coat weight of the second coat is about from two to four pounds per ream.

12. The method of claim 1 wherein the particle size of the calcium carbonate in said third coat is within the range of about from one to 24 microns.

13. The method of claim 12 wherein the solids of said third coat comprise from about 50% to about 80% of carboxylated polyvinyl acetate, with the remainder as calcium carbonate.

14. A method for treating a web of paper to produce electrostatic recording paper comprising the steps of ap plying a first coat of about from one to two pounds per ream to one surface of the web containing an aqueous mixture of a water soluble conductive material and titanium dioxide, applying a second coat over said first coat of about from two to four pounds per ream of an aqueous mixture of starch and calcium carbonate having a particle size of about from one to 24 microns, applying a coat of about from one to two pounds per ream of an aqueous mixture of a conductive material and titanium dioxide to the other surface of the web, applying a third coat over said second coat of about from two to 4 and one-half pounds per ream of an aqueous mixture of a water soluble dielectric material and calcium carbonate having a particle size of from about one to 24 microns.

15. The method of claim 14 wherein the water soluble conductive material is polyvinyl benzyl trimethyl ammonium chloride, and the dielectric material is car- -boxylated polyvinyl acetate in ammoniacal solution.

16. The method of claim 14 wherein the web is dried after the complete coating thereof and the moisture content therein is raised up to about 7%.

17. An electrostatic recording paper comprising a paper base having opposed surfaces, at first coating on a first surface of said base containing titanium dioxide and a conductive material, a second barrier coat over said first coat containing starch and calcium carbonate having a particle size of about from one to 24 microns, and a third coat over said second coat containing a dielectric material and calcium carbonate having a particle size of about from one to 24 microns.

18. The product of claim 17 wherein the conductive material is polyvinyl benzyl trimethyl ammonium chloride, and the dielectric material is the deposited residue of a polyvinyl acetate polymer.

19. The product of claim 17 comprising a coating on the other surface of said base containing titanium dioxide and a conductive material.

20. The product of claim 19 wherein said first surface is machined glazed and said other surface is unglazed.

21. In an electrostatic recording paper having a base sheet with a conductive coating containing titanium dioxide and an outer dielectric layer containing calcium carbonate, the improvement comprising a barrier layer between the base sheet and the: dielectric layer, said barrier layer being compatible. with said conductive coating and dielectric layer and consisting essentially of starch and calcium carbonate having a particle size up to about 24 microns.

References Cited UNITED STATES PATENTS 3,110,621 11/1963 Doggett et al 117218 3,640,708 2/1972 Humphriss et al. 117-201 X 3,481,775 12/1969 Albertson 117201 3,620,828 11/1971 Werdouschegg 117-20l 3,652,268 3/1972 Rowe 117-201 X 3,011,918 12/1961 Silvernail ct al 117-201 3,264,137 8/1966 Gess 117--201 RALPH HUSACK, Primary Examiner US. Cl. X.R.

ll768 76 P, 201, 221 

